Updated September 25, 2026. Fact-checked against veterinary sources; review by a licensed veterinarian is in progress. See our editorial policy.
Quick answer: Mast cell tumors (MCTs) are the most common skin cancer in dogs. They can look like almost anything: a small bump, a fatty-feeling lump, or a red, itchy swelling that changes size. A quick needle test (fine needle aspirate) usually identifies them. Treatment is usually surgical removal with a margin of normal tissue. Many low-grade tumors are cured with surgery, while higher-grade tumors may need radiation, chemotherapy, or targeted drugs.
What they look like
- Raised skin lumps, often hairless and sometimes red
- Soft lumps under the skin that can feel like fatty lumps
- Lumps that swell, shrink, or get itchy, sometimes after being touched
- Anywhere on the body, including legs, trunk, and around the genitals
Breeds at higher risk include Boxers, Boston Terriers, Pugs, Bulldogs, Labrador and Golden Retrievers, and Shar-Peis, but any dog can develop them.
Why MCTs cause symptoms
Mast cells contain histamine and other substances. When a tumor is handled or releases these, it can cause redness, swelling, itching, and in some dogs, stomach ulcers, vomiting, or dark stools.
Diagnosis
- Fine needle aspirate: usually identifies an MCT quickly. See what to expect from a fine needle aspirate.
- Biopsy after removal: determines the grade (low or high) and whether margins are clean.
- Staging: checking nearby lymph nodes, and sometimes abdominal ultrasound, to look for spread.
- Additional tests such as c-KIT mutation testing can guide treatment.
Treatment
- Surgery with wide margins is the main treatment. Medications like antihistamines and acid reducers are often given around surgery.
- Radiation therapy when a tumor can’t be fully removed, such as on a leg.
- Chemotherapy (such as vinblastine with prednisone) for high-grade or spread tumors.
- Targeted drugs such as toceranib.
- Tigilanol tiglate: an FDA-approved injection that treats some non-spread skin MCTs without surgery.
- Supportive medications: antihistamines and stomach protectants to control histamine effects.
Outlook
- Low-grade tumors fully removed: often cured.
- High-grade tumors or tumors that have spread: more guarded, but treatment can control disease and extend good-quality life.
- Dogs that had one MCT can develop new ones, so regular skin checks matter.
Home care
- Check your dog’s skin monthly and note new lumps with photos and measurements.
- Avoid squeezing or rubbing known MCTs.
- After surgery, use a cone and restrict activity as directed.
- Call your vet for vomiting, black stool, or sudden swelling around a tumor.
What to avoid
- Waiting on any new lump. The Veterinary Cancer Society recommends checking lumps pea-sized or larger that have been present for a month.
- Unproven supplements or salves (such as “black salve”), which can cause severe burns.
- Combining steroids and NSAIDs.
Frequently asked questions
Are mast cell tumors always cancerous?
Yes, they’re considered malignant, but low-grade tumors often behave mildly and are cured with surgery.
Can a mast cell tumor go away on its own?
They can shrink temporarily, but they don’t disappear. That’s why testing matters.
Sources
- Merck Veterinary Manual: Mast cell tumors
- Kiupel M, et al. Proposal of a 2-tier histologic grading system for canine cutaneous mast cell tumors to more accurately predict biological behavior. Vet Pathol. 2011
- London CA, et al. Multi-center, placebo-controlled, double-blind, randomized study of oral toceranib phosphate (SU11654) for the treatment of dogs with recurrent (either local or distant) mast cell tumor. Clin Cancer Res. 2009
- US FDA: Approval of Stelfonta (tigilanol tiglate) for mast cell tumors in dogs. 2020
- Veterinary Cancer Society: See Something, Do Something campaign
This article is for general education and is not a substitute for care from your own veterinarian. Learn about The Pet Vet and our editorial policy.